A lithography technique for reducing and transferring various patterns formed on a mask onto a wafer with light beams is used to manufacture devices such as a semiconductor device and the like. A mask pattern for use in lithography is required to have an extremely high degree of accuracy. Hence, to form a mask pattern, an electron beam exposure apparatus is employed. An electron beam exposure apparatus is also employed to directly draw a pattern on a wafer without any masks.
Electron beam exposure apparatuses include, e.g., a point-beam type apparatus which uses spot-like beams and a variable rectangular beam type apparatus which uses beams each having a variable-size rectangular cross section. A general electron beam exposure apparatus of either type comprises an electron gun which generates electron beams, an electron optical system for guiding electron beams emitted from the electron gun onto a sample, a stage system for performing scan driving for the sample to draw a pattern on the entire sample with electron beams, and an objective deflector for positioning electron beams on the sample at high accuracy.
A region in which the objective deflector can position electron beams is designed to have a width of about several mm in order to minimize any aberration in the electron optical system. When a silicon wafer is employed as the sample, its diameter is about 200 to 300 mmφ. On the other hand, when a glass substrate to be used as a mask is employed, its size is about 150 mm square. For this reason, the electron beam exposure apparatus has a stage which can perform scan driving for the sample to draw a pattern on the entire sample with electron beams.
The stage is arranged in a vacuum chamber. The stage is required not to cause any variation in magnetic field, which may affect the positioning of electron beams. For this reason, a contact actuator such as a ball screw actuator is used in a conventional stage.
Conventionally, an increase in speed has been demanded for lithography. For example, Japanese Patent Laid-Open No. 9-330867 discloses a multi electron beam exposure apparatus which irradiates the surface of a sample with a plurality of electron beams in accordance with design coordinates and scans the sample surface while deflecting the plurality of electron beams in accordance with the design coordinates and individually turning on/off the plurality of electron beams in accordance with a pattern to be drawn. A multi electron beam exposure apparatus can draw a pattern with a plurality of electron beams and thus can increase the throughput.
FIG. 6 is a view showing the outline of a multi electron beam exposure apparatus. Electron guns 501a, 501b, and 501c can individually turn on/off electron beams. A reduction electron optical system 502 reduces and projects a plurality of electron beams from the electron guns 501a, 501b, and 501c onto a wafer 503. A deflector 504 scans the plurality of electron beams to be reduced and projected onto the wafer 503.
FIG. 7 shows how the multi electron beam exposure apparatus in FIG. 6 scans a wafer with a plurality of electron beams. White circles represent beam reference positions (BS1, BS2, and BS3) at which each electron beam comes incident on the wafer when it is not deflected by the deflector 504. The beam reference positions are plotted along a design orthogonal coordinate system (Xs,Ys). The respective electron beams scan exposure fields (EF1, EF2, and EF3) for the respective electron beams in accordance with the design orthogonal coordinate system (Xs,Ys) with reference to the beam reference positions. The exposure fields are arranged adjacent to each other, so that a larger pattern can be drawn.
The positioning responsiveness of electron beams is extremely high. For this reason, instead of an arrangement for improving the mechanical control characteristics of a stage, there is generally employed an arrangement for adjusting the incident positions of electron beams with respect to a wafer by measuring the posture and positional shift amount of the stage and controlling a deflector for scanning the electron beams on the basis of the measurement result, as disclosed in, e.g., Japanese Patent Laid-Open No. 5-89815. This method, however, is based on the premise that the positional relationship between a wafer to be exposed and a measuring mirror used to measure the posture and positional shift amount of the stage remains unchanged. For example, if a structure is distorted by an external force to cause fluctuations in relative position between the measuring mirror and wafer, a pattern error may occur.
In a conventional single-beam exposure apparatus, a focus error (fluctuations in posture) in a stage causes no serious problem. On the other hand, in a multi electron beam exposure apparatus which uses a plurality of electron beams, Z-direction adjustment and posture adjustment (a tilt mechanism) are required to position each electron beam within a predetermined focus tolerance. An increase in the number of degrees of freedom in adjustment increases the number of actuators. The use of an actuator having high rigidity such as a contact actuator is highly disadvantageous in that a structure is distorted by a driving reaction force.
An electromagnetic actuator can implement a non-contact arrangement having no rigidity and can solve problems of a driving reaction force and dust. In electron beam exposure, any fluctuations in magnetic field are not allowed even if they are small. Fluctuations in magnetic field can be reduced by arranging an electromagnetic actuator at a position remote from a substrate-bearing surface and providing a multiple shield in the electromagnetic actuator. Therefore, the use of an electromagnetic actuator presently attracts attention.
If an electromagnetic actuator is to be employed as an actuator for stage driving in an electron beam exposure apparatus, the electromagnetic actuator must be arranged at a position remote from a substrate-bearing surface, as described above. For this reason, if position measurement for stage control is performed using a mirror arranged on the substrate-bearing surface, vibrations having various natural frequencies occur in a control system. The control gain cannot be set to a high value, thus resulting in difficulty in high-speed and stable control of a stage. Conventionally, this makes it difficult to draw a pattern on a substrate at high speed and high accuracy.